This invention relates to valves and, more particularly, to pilot-operated spool valves of the type frequently utilized in controlling the flow of hydraulic fluid to the hydraulic motors driving the tracks on crawler-type vehicles.
Increasingly, crawler-type vehicles are being constructed with hydraulic motors for providing propulsion. Typically, one hydraulic motor will be provided for each track and the flow of hydraulic fluid to each motor is individually controlled to provide turning movements, etc., by varying the speed and/or direction of movement of one track with respect to the other.
However, since the propulsion systems for the different tracks are completely independent, a problem attends their use if there is mismatching of the corresponding components. In such a case, for identical positions of the controls for each of the tracks, the vehicle will not travel in a straight line due to more resistance in one system than in the other, which resistance may be either hydraulic or mechanical, or both.
Frequently, the problem is minimal when maximum hydraulic pressure is applied to both propulsion systems. However, there are many uses of such vehicles wherein the maximum speed is seldom used. Rather, lesser speeds are more frequently used with the result that the mismatching will cause the above mentioned difficulty.
It is, of course, impossible at the time of manufacture to know whether the vehicle will be principally utilized at maximum speed or at some lesser speed so that an adjustment to the system can be made at that time. Thus, adjustments frequently must be made in the field, requiring the presence of trained personnel and the attendant expense.